1. Field of the Invention
The present invention relates to polyester molding compositions which exhibit improved rates of crystallization. In particular, the invention relates to molding compositions of polyethylene terephthalate having an increased crystallization rate even at low mold temperatures, a good combination of mechanical and thermal properties, and an improved hydrolytic stability.
2. Description of the Background
Polyethylene terephthalate (PET) is used extensively in the manufacture of fibers, films and containers (bottles) because of its low cost, good physical properties and good electrical properties. However, PET cannot be employed in injection molding processes because the crystallization rate of the PET is slow and, as a consequence, the polymer does not crystallize completely in the mold. At very low mold temperatures, i.e., about 27.degree. F., the PET can be injection molded into various shaped articles which have a completely amorphous structure. However, the dimensions of such molded parts or objects will change when the objects are heat aged at temperatures above the glass transition temperature (Tg) of the PET, which results in reduced dimensional stability of the molded parts. When unmodified PET is molded at mold temperatures above the Tg of the polymer, the high viscosity of the polymer does not permit the polymer to crystallize to a sufficient extent. Rather, in such cases, the surface of the polyester in the mold sticks to the mold surfaces, thus making it difficult to eject the molded article from the mold. In fact, in such molding very long cycle times and high mold temperatures are required in order to successfully process unmodified PET in the injection molding process.
It is known that the crystallization rate and crystallization temperature of PET can be increased by the addition of a nucleating agent to a molding composition. Several nucleating agents have the characteristic of promoting rapid crystallization under conditions of rapid cooling from the melt. An acceptable nucleating agent should be highly effective at low concentration in promoting crystallization, and further should be readily dispersable so that rapid and uniform crystallization of the polymer from the melt is possible.
Commonly employed nucleating agents for PET are the alkali and alkaline earth metal salts of organic acids. U.S. Pat. No. 3,352,904 discloses the sodium and potassium salts of carboxylic acids, as well as the salts of organic polymers containing pendant acidic groups, including carboxylic acid groups, as nucleating agents for PET. U.S. Pat. Nos. 4,322,335; 4,368,288; 4,305,864; 4,390,493; 4,393,178 and EP 21,648 and 351,732 all disclose the use of ionizable metal salts of aromatic ring containing compounds, wherein the aromatic rings are substituted by acid groups which are carboxylic acid, sulfinic acid, phosphonic acid, phosphinic acid and phenolic groups.
U.S. Pat. No. 3,575,931 shows metal salts of carboxylic acids as nucleating agents for PET. U.S. Pat. No. 4,401,792 discloses alkali metal salts of polyethylene ionomers and benzoic acid as nucleating agents for PET. U.S. Pat. No. 4,425,470 discloses alkali metal salts of ethylene terephthalate oligomers as nucleating agents for PET. U.S. Pat. No. 4,551,507 shows alkali and alkaline earth metal carboxylate salts of tertiary and quaternary amines as PET nucleating agents.
The crystallized polyesters obtained by the art known processes do not show sufficient heat resistance and hydrolytic stability in many applications where the molded polymers are exposed to humid conditions and high heat. Still another drawback of the nucleated polyester compositions known to the art is that the polymer during its processing suffers a decrease in weight average molecular weight, mostly because of the degradation phenomenon caused by ionic nucleants. This effect is also responsible for the observed decrease in the mechanical properties of the polyesters. Further, small molecular weight acids remaining in the polymer after nucleation which are derived from the acid salts cause acid degradation of the PET. A need therefore continues to exist for multiple polyester compositions having enhanced crystallization rates along with good hydrolytic stability and mechanical properties.